Abstract: In infrared detection systems, when the target signal is relatively weak, the noise caused by a spot of stray light will greatly reduce the SNR of system, thus reduce the system's detection capability. For highly sensitive infrared imaging system and low temperature environment simulation system, the thermal radiation emitted from the optical surfaces is the largest source of stray light. In this paper, the certainty and near-field property of self-emitted thermal radiation stray light were analyzed, and a new computational model based on ray optics was proposed. Some key problems in the calculation model were discussed, the solutions were proposed. Moreover, the near-field thermal radiation of objects was accurately numerical simulated. Distribution of irradiance on the detector was gained, the effects of temperature, distance, radiation coefficient and other parameters on the stray light level were analyzed, which has a guiding significance on reducing the system' s thermal radiation noise reasonably and efficiently.